Wide range constant concentration particle generating system

ABSTRACT

A particle generating system includes an aerosol generator, an ejector diluter, and an aerosol diluter. The ejector diluter receives the generated aerosol and dilutes the aerosol to an expected raw concentration. The aerosol diluter further dilutes the aerosol to a concentration in the range of 0% to 100% of the expected raw concentration. The aerosol diluter includes a mini cyclone for diluting the aerosol. The particle generating system may be configured to provide variable concentrations of monodisperse or polydisperse aerosols for instrument calibration. The system may provide constant concentrations in the range of 0% to 100% of the raw concentration. The mini cyclone makes the system compact, and the system may be portable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to particle generating systems and to calibratingparticle instruments.

2. Background Art

Particle instruments have been widely applied to detect particulatematter level (mass and number) in ambient air, specific environments,and combustion engines, etc. To ensure that these instruments performaccurately, frequent calibrations with different constant concentrationaerosols are extremely necessary.

Currently, aerosol generators, such as atomizer and propane burner,etc., have been widely used to generate particles. Many different typesof diluters have been applied to dilute particles to differentconcentrations as well. However, the dilution ratio range is narrow, anddoes not provide concentration in the range of 0% to 100%.

By combining these two techniques, calibration aerosol is available tocalibrate particle instruments. Since aerosol generators and dilutersare in separate units, the units must be put together correctly togenerate the expected concentration aerosol. The setup procedure istime-consuming, and not efficient. Many variations may be involvedduring the setup. As a result, greater uncertainties may be introducedinto the calibrated instrument.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide an improved particlegenerating system.

The invention involves generating well known concentrations of mono- orpolydisperse aerosol for instrument calibration. In a preferredimplementation, accurate aerosol concentration is available in the rangeof 0% to 100% of the raw aerosol by diluting the raw aerosol.

In accordance with the invention, a wide range constant concentrationparticle generating system provides calibration aerosol with well knowncharacteristics. In one aspect, the invention may involve integrating anaerosol generator, ejector diluter, and aerosol diluter into a singlesystem.

In an exemplary approach to carrying out the invention, an ejectordiluter dilutes aerosol to the expected raw concentration. An aerosoldiluter dilutes the aerosol further to a concentration in the range of0% to 100% of the raw concentration. According to the invention, a minicyclone on the aerosol diluter is used to mix aerosol, which dilutes rawaerosol from the ejector diluter and aerosol generator. Since thecyclone results in low particle losses and the aerosol diluter providesaccurate dilution ratios, accurate concentration in the expected sizerange and similar size distributions are obtained in the range of 0% to100% of the raw aerosol concentration. In this particular approach, thecyclone removes large size particles as well. And so as a result, thisprotects calibrated instruments from malfunctions. By using the minicyclone instead of a traditional tunnel for mixing, the actual size ofthe diluter may be reduced. A PID loop controls the dilution ratio asconstant on the aerosol diluter when a constant dilution ratio andconcentration are expected.

The aerosol generator generates polydisperse aerosol under mostcircumstances. By feeding the aerosol generated by the aerosol generatorinto a size instrument, such as a differential mobility analyzer (DMA),monodisperse aerosol with 0% to 100% of raw concentration becomesavailable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a flow schematic for a wide range constant concentrationparticle generating system that provides polydisperse aerosol forcalibration in accordance with a preferred embodiment of the invention;

FIG. 2 depicts a flow schematic for a wide range constant concentrationparticle generating system that provides monodisperse aerosol forcalibration in accordance with a preferred embodiment of the invention;

FIG. 3 illustrates an ejector diluter;

FIG. 4 illustrates an aerosol diluter; and

FIG. 5 illustrates an alternative design of the aerosol diluter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIGS. 1 and 2, in the preferred embodiment, the wide rangeconstant concentration particle generating system includes aerosolgenerator 10, aerosol conditioning unit 12, neutralizer 14, highefficiency particle filter (HEPA) 16, ejector diluter 18, aerosoldiluter 20, and control system 22. The system also includes a suitablevacuum source, and particle-free compressed air, etc. FIGS. 1 and 2 showthe flow schematic of the system for poly- and monodisperse aerosol,respectively.

In FIG. 1, aerosol generator 10 generates aerosol by atomizing a liquidsolution with compressed air, or combusting propane or diesel fuel on aburner, or other means. The type of generator used is determined by thecalibrated instruments and their applications. For example, if theinstrument is a condensation particle counter, which measures theparticle number concentration, the atomizer is a good choice for thegenerator.

After aerosol flows into conditioning unit 12, water or liquid drops orvapor is removed. Then, the aerosol moves into neutralizer 14. Byadjusting needle valve 15, the extra flow can be vented from HEPA filter16, where particles in aerosol are removed, if the flow required inejector diluter 18 is less than that generated by aerosol generator 10.Under some circumstances, the flow is sucked into ejector 18 throughHEPA filter 16, where particles in ambient air are removed, if generator10 does not generate enough flow for ejector diluter 18. With an inletopen on HEPA filter 16, the flow pressure in the aerosol can bestabilized.

In neutralizer 14, the aerosol is charged to Boltzmann equilibrium. As aresult, particle losses, which are caused by static charges onparticles, are reduced. The aerosol is diluted in ejector diluter 18 tothe expected concentration. Partial flow from ejector 18 moves intoaerosol diluter 20. A large fraction of the aerosol is vented.

When the dilution ratio is 1:1 on aerosol diluter 20, the concentrationof aerosol from ejector diluter 18 is measured by calibrated instrument24. This number is recorded and saved in computer 22 as the rawconcentration of the aerosol. By inputting the expected percentageconcentration from the computer, the computer and PID loop in controlsoftware control the aerosol diluter 20 to the expected dilution ratios.100% concentration means no dilution on the aerosol, and 0%concentration means no aerosol into the calibrated instrument 24.

In FIG. 2, the aerosol flowing from neutralizer 14 is connected to adifferential mobility analyzer (DMA) 26 instead of directly to ejectordiluter 18. DMA 26 can output single size (monodisperse) particles byrunning at constant voltage. The monodisperse aerosol flows into ejectordiluter 18, which functions to vent or compensate the flow from DMA 26while the aerosol from DMA 26 is higher or lower than that expected.Except for these noted differences, operation of the system formonodisperse aerosol is the same as operation of the system forpolydisperse aerosol.

FIG. 3 illustrates the flow schematic of ejector diluter 18 in moredetail. The flow schematic includes ejector 30, orifice 32, pressureregulator 34, and pressure gauge 36, HEPA filter 38, as well as theparticle free compressed air and by-pass.

Ejector 30 is operated by particle free compressed air. When compressedair flows through ejector 30, vacuum is generated at the inlet side ofejector 30. The vacuum sucks the aerosol flow, which is from neutralizer14 or DMA 26, into the ejector. Aerosol is mixed with particle freecompressed air quickly and uniformly in the ejector. Most of the mixturefrom ejector 30 is vented, and a small fraction of the mixture flowsinto the aerosol diluter.

With a specific size orifice 32, different dilution ratios can beobtained by adjusting the pressure of the compressed air. Under mostcircumstances, the greater the compressed air pressure is, the higherthe dilution ratio is. Put another way, the lower the compressed airpressure is, the lower the dilution ratio is.

The size of orifice 32 is the other major factor to adjust dilutionratio on ejector 30. With a larger size orifice, a smaller dilutionratio can be obtained. Put another way, a greater concentration of theaerosol can be obtained. With a smaller size orifice, a greater dilutionratio and lower aerosol concentration can be obtained.

In the case where polydisperse aerosol is expected, ejector diluter 30receives the aerosol from neutralizer 14 directly. HEPA filter 38 shouldbe closed by plug 40, because HEPA filter 16 and needle valve 15(FIG. 1) upstream of the neutralizer can ensure the right amount of flowinto the ejector diluter by venting or sucking extra flow.

In the case where monodisperse aerosol is expected, the aerosol fromneutralizer 14 moves into differential mobility analyzer (DMA) 26 (FIG.2). DMA 26 selects single size particles by running at a fixed columnvoltage. A column voltage is related to a specific particle size. DMA 26outputs constant air flow as well. This flow may be greater or less thanthat required by ejector diluter 18.

With continuing reference to FIG. 3, by taking off the plug 40 connectedto HEPA filter 38 on the ejector diluter, the flow into the ejectordiluter can be adjusted automatically. For example, when the DMA is notable to provide enough flow to the ejector diluter, ambient air filteredby the HEPA filter 38 moves into and mixes with the aerosol from theDMA; when the DMA provides more flow than that required by the ejectordiluter, the extra flow from the DMA is vented through the HEPA filter38. As a result, the adjustment of the dilution ratio on the ejectordiluter does not influence the performance of the DMA.

FIG. 4 illustrates the flow schematic of the aerosol diluter in moredetail. This includes mass flow controller 60, mass flow controller 62,mini cyclone 64, and vacuum pump 66.

Aerosol from the ejector diluter moves into aerosol diluter 20, anduniformly mixes with particle free compressed air in mini cyclone 64.Particles larger than 2.5 micrometers are removed by cyclone 64, andcyclone 64 protects the calibrated instrument from malfunction caused bylarge size particles. Flow rates of the dilution air and total flow arecontrolled by the two mass flow controllers 60, 62. The computersoftware and hardware control these flow rates to obtain the expecteddilution ratio or aerosol concentration. The well known flow rate ofaerosol moves into the calibrated instrument 24. The extra flow isevacuated by vacuum pump 66.

The following equations show the calculation of the dilution ratio andconcentration:${Dr} = \frac{Q_{totalflowrate} + Q_{instrument}}{\left( {Q_{totalflowrate} + Q_{instrument}} \right) - Q_{dilutionair}}$$C = {\frac{C_{raw}}{Dr} = {p*C_{raw}}}$ $p = \frac{1}{Dr}$

Where, Q_(totalflowrate) is total flow through the flow controller;Q_(instrument) is well defined flow rate to the calibrated instrument;Q_(dilutionair) is the dilution air flow rate; C_(raw) is aerosolconcentration from the ejector diluter; Dr is the dilution ratio on theaerosol diluter; C is expected concentration; p is the percentageconcentration in 0 to 100%. All flow rates above are at standardcondition or the same reference condition.

To have 100% concentration, the dilution air flow is zero. As a result,raw aerosol from the ejector only moves into the cyclone. To have 0%concentration aerosol into the calibrated instrument, Q_(dilutionair)should equal to or be larger than Q_(totalflowrate)+Q_(instrument) inthe above equations. As a result, no aerosol flow moves into the aerosoldiluter.

When the constant concentration of the aerosol is expected, the dilutionratio on the aerosol diluter needs to keep as constant. A PID loop(FIGS. 1 and 2) has been built to control the dilution ratio at theconstant. By comparing the set point of the dilution ratio or thepercentage concentration to the real value, the PID loop adjusts theflow rate of the dilution air. As a result, constant dilution ratio ismaintained.

FIG. 5 shows the alternative design of the aerosol diluter at 70. Thecritical orifice 72 and a mass flow meter 74 replace the mass flowcontroller 62 shown in FIG. 4. This provides the same function as themass flow controller for the total flow control. By changing the size ofthe critical orifice 72, different total flows can be obtained.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A particle generating system comprising: an aerosol generator forgenerating aerosol; an ejector diluter receiving the aerosol anddiluting the aerosol to an expected raw concentration; and an aerosoldiluter receiving the diluted aerosol and further diluting the aerosolto a concentration in the range of 0% to 100% of the expected rawconcentration, wherein the aerosol diluter includes a mini cyclone fordiluting the aerosol.
 2. The system of claim 1 further comprising: aconditioning unit between the aerosol generator and the ejector diluter,the conditioning unit removing vapor from the generated aerosol.
 3. Thesystem of claim 1 further comprising: a neutralizer between the aerosolgenerator and the ejector diluter, the neutralizer charging the aerosolto Boltzmann equilibrium.
 4. The system of claim 1 further comprising: ahigh efficiency particulate filter accommodating flow between ambientand the ejector diluter such that flow from the aerosol generator can bevented from the filter or flow can be drawn through the filter to theejector diluter, depending on the required flow of the ejector diluter.5. The system of claim 1 further comprising: a PID loop controlling adilution ratio for the aerosol diluter.
 6. The system of claim 5 whereinthe PID loop controls the dilution ratio as constant.
 7. The system ofclaim 1 further comprising: a size instrument receiving the generatedaerosol from the aerosol generator and producing a monodisperse aerosolfor reception by the ejector diluter.
 8. The system of claim 7 whereinthe size instrument is a differential mobility analyzer.
 9. The systemof claim 1 wherein the aerosol generator generates polydisperse aerosolthat is received by the ejector diluter.
 10. The system of claim 1wherein the aerosol diluter further comprises: a first mass flowcontroller connecting a particle-free source to the mini cyclone fordiluting the aerosol; a vacuum pump; a second mass flow controllerconnecting the mini cyclone to the vacuum pump; and an outlet betweenthe second mass flow controller and the mini cyclone for connecting toan instrument.
 11. The system of claim 1 wherein the aerosol diluterfurther comprises: a first mass flow controller connecting aparticle-free source to the mini cyclone for diluting the aerosol; avacuum pump; a mass flow meter and critical orifice between the minicyclone and the vacuum pump; and an outlet between the second mass flowcontroller and the mini cyclone for connecting to an instrument.